Beyond the Shell ModelShort-Range Nucleon-Nucleon Correlations

byDouglas W. Higinbotham, Jefferson Lab

[1] R. Subedi et al., Science 320 (2008) 1476.[2] R. Shneor et al., Phys. Rev. Lett. 99 (2007) 072501.[3] E. Piasetzky et al., Phys. Rev. Lett. 97 (2006) 162504.[4] K. Egiyan et al., Phys. Rev. Lett. 98 (2007) 262502.[5] M. Sargsian et al., Phys. Rev. C71 (2005) 044615.[6] R. Schiavilla et al., Phys. Rev. Lett. 98 (2007) 132501.[7] T. Frink et al., Phys. Rev. C71 (2005) 014313.

Introduction

Tensor Force

Results Neutron Stars

References

The structure of nuclei is determined by the nature of the strong force: strong repulsion at short distances and strong attraction at moderate distances. This force, which binds the nucleons in the nucleus while also keeping them from collapsing, makes the nucleus a fairly dilute system. This allowed calculations that treated the nucleus as a collection of hard objects in a mean field to describe many nuclear properties. Of course, this simple picture of the nucleus is inaccurate, as the nucleons should be thought of as waves that can strongly overlap for short periods of time.

ExperimentThe cartoon shows, in momentum space, the momentum distribution of protons and neutrons in neutron stars. The figure on the left shows the case where the protons and neutrons weakly interact and can be approximated as separate Fermi spheres. The figure on the right shows how strong neutron-proton short-range correlations cause the protons to escape out of their Fermi sphere and have much higher momentum [7].

Shown are the fractions of short-range correlated pair combinations in carbon as obtained from the Jefferson Lab (e,e’pn) and (e,e’pp) reactions [1,2], as well as from Brookhaven (p,2pn) data [3]. These results also agree with the interpretation of recent inclusive data [4].

Correlated initial states are probed by selecting kinematics beyond the Fermi momentum of nucleons in the nucleus and far from the delta resonance. As shown for a correlated pair, knocking out one nucleon causes a high momentum correlated partner nucleon to be emitted from the nucleus.

The results show the complete dominance of proton-neutron pairs over other pair types. Calculations explain the magnitude of the neutron-proton to the proton-proton ratio as being due to the short-range tensor part, or nucleon-nucleon spin-dependent part, of the nucleon-nucleon force [5,6].

The Hall A high-resolution spectrometers were used to detect the electron and proton of the carbon (e,e’p) reaction while the BigBite spectrometer along with a neutron detector were used to detect correlated partner nucleons.